Conventionally, when producing a 4N or 5N (respectively implying 99.99 wt %, 99.999 wt %) level higher purity metal, the electro-refining method is often employed for the production thereof. Nevertheless, there are many cases where approximate elements remain as impurities when performing electrolysis to the target metal. For example, in the case of a transition metal such as iron, numerous elements such as nickel, cobalt and so on, which are also transition metals, are contained as impurities.
When refining such crude metals of a 3N level, electrolysis is performed upon producing a higher purity liquid.
In order to obtain a higher purity metal in the foregoing electrolysis, it is necessary to employ a method of ion exchange or solvent extraction for producing an electrolytic solution with few impurities.
As described above, the production of an electrolytic solution normally requires a refinement in advance prior to the electrolysis, and has a shortcoming in that the production cost therefor would become high.